Microlithography is used for producing microstructured components, such as for example integrated circuits or LCDs. The microlithography process is carried out in what is called a projection exposure apparatus, which includes an illumination device and a projection lens. The image of a mask(=reticle) illuminated via the illumination device is in this case projected via the projection lens onto a substrate (for example a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In a projection exposure apparatus designed for EUV (for example for wavelengths of for instance approximately 13 nm or approximately 7 nm), mirrors are used as optical components for the imaging process because of the unavailability of light-transmissive materials. These mirrors may for example be mounted on a supporting frame and be designed as at least partially manipulable, in order to allow a movement of the respective mirror in six degrees of freedom (i.e. with respect to displacements in the three spatial directions x, y and z and also with respect to rotations Rx, Ry and Rz about the corresponding axes). This allows compensations to be made for changes in the optical properties that occur for instance during the operation of the projection exposure apparatus, for example as a result of thermal influences.
It is for example known to use in a projection lens of an EUV projection exposure apparatus for the manipulation of optical elements such as mirrors in up to six degrees of freedom—as schematically indicated in FIG. 4—three actuator arrangements, which respectively include at least two Lorentz actuators 402 and 403, 404 and 405 and also 406 and 407 (i.e. two actively activatable axes of movement in each case). Also provided in the construction from FIG. 4 for each of these actuator arrangements or for each associated point of force introduction there is in each case a weight compensating device bearing the weight of an optical element or mirror 400, in order to minimize the energy consumption of the active or controllable adjusting elements, so that in this respect no permanent energy flow with accompanying heat generation is required. The weight compensating device, also referred to as “MGC”(=“Magnetic Gravity Compensator”), can be set to a certain holding force, which is transmitted to the mirror 400 by a mechanical element (pin) 415, 425 or 435 that is mechanically coupled to the mirror 400.
According to FIG. 5, the magnetic circuit itself conventionally includes a (passive) magnetic circuit made up of an outer magnetic ring 510, which in the example represented is polarized radially with respect to the z axis extending in the driving direction, and two magnetic rings 521, 522 arranged radially further inward, which in the exemplary embodiment represented are respectively polarized axially with respect to the z axis, both the outer magnetic ring 510 and the inner magnetic rings 521, 522 being formed in each case as permanent magnets. This assembly is guided by way of a linear guide, formed by a system of parallel springs including leaf springs 531, 532. The pin mechanically coupled to the mirror (denoted in FIG. 5 by “500”) or a bearing bush 501 formed on the latter in the example of FIG. 5, is denoted in FIG. 5 by “540” and for its part includes two flexures 541, 542 formed as leaf spring joints, by way of which a flexible attachment to the mirror 500 in all directions apart from the axial z direction is achieved.
However, with the mechanical attachment of a weight compensating device described above by way of a pin according to FIG. 5, there is the issue that the pin 540 itself has dynamic vibration modes on account of possible bending movements of the pin. These internal vibration modes may on the one hand impair the stability of the positional control of the mirror concerned and on the other hand impair the effectiveness of the desired mechanical decoupling in the higher frequency range, with the consequence that the sensitivity of the mirror with respect to disturbances in the higher frequency range increases.
One way of overcoming the impairment of the dynamic behaviour described above includes eliminating a mechanical attachment in the form of a pin by using magnetic forces for the coupling, the pin being replaced by an (air) gap and the radially inner magnetic rings 521, 522 from FIG. 5 being mounted directly on the mirror 500. However, a resultant desire to dispense with any mechanical guidance then has the disadvantage that, during a typically intended movement of the mirror taking place in six degrees of freedom, varying distances between the magnetic rings forming the passive magnetic circuit have the effect that comparatively high parasitic forces are transmitted to the mirror by the weight compensating device, which in turn leads to undesired deformations of the optically effective surface of the mirror concerned, and consequently to an impairment of the performance of the optical system. Minimizing these parasitic forces by optimizing the design of the passive magnetic circuit has proven to be difficult to realize here because of unavoidable magnetic and geometrical tolerances.
Furthermore, the magnetic coupling described above also represents a demanding challenge to the extent that, to avoid the introduction of parasitic moments to the mirror and accompanying deformations, the force transmission through the magnetic circuit has to take place as close as possible to the neutral plane of the mirror, which in turn can prove to be difficult from aspects of installation space.
Reference is made merely by way of example to DE 10 2009 054 549 A1 and the publication R. Deng, R. Saathof, J. W. Spronck, S. Hol, R. Munnig Schmidt: “Integrated 6-DoF Lorentz Actuator with Gravity Compensator for Precision Positioning”, 2014, Proc. 22nd Intl. Conf. on Magnetically Levitated Systems and Linear Drives.